Re: [PATCH] swapfile : fix the wrong return value

[Hugh Dickins <hugh.dickins@tiscali.co.uk>]

On Tue, 2 Mar 2010, Huang Shijie wrote:

> If the __swap_duplicate returns a negative value except of the -ENOMEM,
> but the err is zero at this time, the return value of swap_duplicate is
> wrong in this situation.
> 
> The caller, such as try_to_unmap_one(), will do the wrong operations too
> in this situation.
> 
> This patch fix it.
> 
> Signed-off-by: Huang Shijie <shijie8@gmail.com>
> ---
>  mm/swapfile.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 6c0585b..191d8fa 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -2161,7 +2161,7 @@ int swap_duplicate(swp_entry_t entry)
>  {
>  	int err = 0;
>  
> -	while (!err && __swap_duplicate(entry, 1) == -ENOMEM)
> +	while (!err && (err = __swap_duplicate(entry, 1)) == -ENOMEM)
>  		err = add_swap_count_continuation(entry, GFP_ATOMIC);
>  	return err;
>  }
> -- 

I was on the point of Ack'ing your patch, and despairing at my confusion,
when I realized what's actually going on here - the key is (look at 2.6.32)
swap_duplicate() used to be a void function (no error code whatsoever),
until I added the -ENOMEM for swap_count_continuation.  And in fact your
patch is wrong, copy_one_pte() does not want to add swap_count_continuation
in the case when it hits a corrupt pte (one which looks like a swap entry).

But you're absolutely right that it cries out for a comment:


[PATCH] mm: add comment on swap_duplicate's error code

swap_duplicate()'s loop appears to miss out on returning the error code
from __swap_duplicate(), except when that's -ENOMEM.  In fact this is
intentional: prior to -ENOMEM for swap_count_continuation, swap_duplicate()
was void (and the case only occurs when copy_one_pte() hits a corrupt pte).
But that's surprising behaviour, which certainly deserves a comment.

Reported-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
---

 mm/swapfile.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- 2633/mm/swapfile.c	2010-02-24 18:52:17.000000000 +0000
+++ linux/mm/swapfile.c	2010-03-04 00:11:35.000000000 +0000
@@ -2155,7 +2155,11 @@ void swap_shmem_alloc(swp_entry_t entry)
 }
 
 /*
- * increase reference count of swap entry by 1.
+ * Increase reference count of swap entry by 1.
+ * Returns 0 for success, or -ENOMEM if a swap_count_continuation is required
+ * but could not be atomically allocated.  Returns 0, just as if it succeeded,
+ * if __swap_duplicate() fails for another reason (-EINVAL or -ENOENT), which
+ * might occur if a page table entry has got corrupted.
  */
 int swap_duplicate(swp_entry_t entry)
 {

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